In the past several years, direct thermal imaging by thermal imaging printers has become a popular method for recording documents and data due to the low cost and reliability of equipment. Infrared imaging is also a convenient and inexpensive way to produce monochrome thermal transparencies for overhead projector presentations. Technology commonly used for direct thermal printing devices is well known and described in U.S. Pat. Nos. 4,289,535 and No. 4,675,705 where colorless or pale colored chromogenic dyestuffs are combined with a color-developing agent such as benzyl-p-hydroxy benzoate or 4,4'-isopropylidenediphenol. This technology, however, is not well suited for the manufacture of single sheet, transparent films for overhead projection presentations. The technology commonly used to produce single sheet, transparent black image films is also well known and is described in several United States patents including U.S. Pat. Nos. 3,080,254, 3,031,329, 3,446,648 and 5,026,606, in which a noble metal salt of an organic acid, preferably silver behenate or silver stearate, is reduced by an incorporated cyclic organic reducing agent such as the methyl ester of gallic acid in the presence of a toning agent to produce a dense black image. The color-forming layer is clarified by the application of polymeric overcoats such as cellulose acetate.
This art also teaches that resin binders suitable for the carrier system of the inventions are only those which are soluble in organic solvents such as methyl ethyl ketone, acetone, and heptane. The use and disposal of organic solvents, however, raises environmental and worker safety concerns. These solvents are inherently flammable or explosive and their use requires specially-adapted and expensive manufacturing equipment. In addition, they are effluent of the manufacturing process and must be recovered or burned, thus adding to the cost of manufacture.
Furthermore, the single sheet transparency compositions commercially available for use in direct thermal printing applications have been found to cause sticking of the imaging material to the print head, and have had insufficient sensitivity or thermal response characteristics to produce an adequately dense black output. In addition, commercially available compositions exhibit low maximum density (D-max), high minimum density (D-min), and high light scatter or haze.
Thus, there exists a continuing need for thermal imaging materials which can be manufactured safely and with no adverse environmental impact; which will produce images of great clarity with little haze, very high maximum density, and low minimum density; and which will not stick to the print head :nor cause melted material to accumulate on the print head.